On oil fields in open sea, situated a long way off-shore, petroleum fluids such as crude oil or gas are generally recovered, processed, and stored on board a said floating support often referred to as an FPSO (floating-production-storage-offloading). Petroleum fluids such as crude oil and/or gas are then exported by offloading ships that call regularly, e.g. every week, to recover the production from the oil fields. Three techniques are commonly used for this purpose.
A first technique consists in installing a buoy a long way from the FPSO, i.e. at about 1000 meters (m) to 1500 m, which buoy is referred to as a loading buoy and is connected to the FPSO by an undersea pipe, with oil or gas being transferred to said loading buoy via said undersea pipe. An offloading tanker then moors on said loading buoy and the load is recovered via flexible connection pipes connected to said buoy floating on the surface of the sea, the oil or gas then being driven by a pump on board the FPSO in order to load the offloading tanker.
A second technique consists in causing the offloading tanker to draw up alongside the FPSO, i.e. side against side. Under such circumstances, transfer is performed either by hinge-type loading arms, as is common practice for transfers in port, or else by flexible pipes of short length.
A third technique consists in placing the offloading ship in tandem with the FPSO, i.e. the offloading ship takes up a position on the axis of the FPSO at a safe distance of at least 50 m to 150 m, and then moors thereto, after which it recovers the ends of floating pipes that extend from the floating support where they are connected to a tank at their ends opposite from their ends floating on the sea, the recovered ends being connected on board said offloading ship, and oil or gas then being driven by a pump from the FPSO in order to load the offloading tanker.
On oil fields, it is generally preferred to use a loading buoy, while nevertheless associating it in general with a redundancy device that may be either an alongside device, i.e. a device in which the offloading ship is arranged alongside the FPSO, or else a tandem device, i.e. a device in which an offloading ship is arranged in tandem with the FPSO, and sometimes even both such devices.
In all configurations, with crude oil, the connection pipes either to a loading buoy or to an offloading ship are kept full, either with crude oil or else with an oil product, generally gasoil, that takes the place of crude oil when there is a risk of the crude oil solidifying (paraffinic crudes).
In contrast, when transferring liquefied gas of the LNG type at −165° C., transfer devices include at least one go connection pipe for the liquefied gas and a return connection pipe, in general of smaller diameter, for removing gas from the tanks of the offloading ship progressively as they are filled with LNG, and in particular for removing methane gas so that it can be reliquefied on board the FPSO. Furthermore, the connecting flexible pipes need to be emptied practically completely after offloading so as to avoid ice forming and accumulating on said pipes and more particularly on the mechanical connections of said pipes. Furthermore, the pipes need to include extremely good insulation so as to limit the amount of liquid methane (LNG) that is regassified during transfer. That is why, for this purpose, it is preferred to use offloading techniques with alongside and tandem devices in which firstly the connecting pipes are not undersea pipes but rather pipes that float on the surface, and secondly said pipes are of relatively short length.
Nevertheless, offloading alongside is very difficult since it is possible to envisage bringing an offloading ship into a position parallel with the FPSO and at a distance of less than 5 m therefrom only under sea conditions that are very clement. With rough sea, transfer becomes impossible, and if the FPSO is full that can make it necessary to stop production, which constitutes a serious handicap for the profitability of installations exploiting the oil fields in question. The transfer means are then constituted either by conventional loading arms, or by devices having flexible pipes installed on board said FPSO. Patent EP-2 239 190 describes such an alongside offloading device.
Tandem offloading presents much greater safety, but offloading pipes are longer and therefore more complicated to handle and store on board the FPSO. For this purpose, numerous devices have been developed for storing and guiding flexible pipes on board an FPSO. Some of them use of a considerable carrier structure of the hinge type supporting a plurality of rigid pipes including rotary joints as hinges, as described in detail in U.S. Pat. No. 4,393,906.
Other solutions include using rigid pipes hinged with rotary joints, or flexible pipes arranged in festoons between the FPSO and the offloading ship, as explained in patent WO 01/04041.
Another technical field is one in which LNG is stored at sea close to a utilization site, e.g. in order to deliver gas to land after it has been regassified, or indeed to transform it on site into electricity for delivering said electricity to the local network. Under such circumstances, the ship comes to unload its cargo of LNG and the floating support is referred to as an FSRU (floating storage regasification unit).